1. Field of the Invention
The invention relates generally to brushless motor speed controls and more specifically to digital speed controls.
2. Brief Description of the Prior Art
Prior brushless motor speed controls have had two basic aspects, a speed sensing technique and a power control technique. Among the speed sensing techniques are:
1. Frequency to voltage conversion involving converting rotor position sensor signals to a voltage proportional to shaft speed and comparing this to a reference signal to develop an error signal;
2. A phase locked loop wherein the rotor position signal is phase compared with a reference signal to develop an error signal; and
3. A digital measurement of rotation time from the rotor position signals which is compared with a reference time period to develop an error signal.
Among the power control techniques are:
1. A linear control wherein the error signal controls a linear regulator of motor drive voltage or current;
2. An on/off control turning the power off at nominal speed or on and off over fixed-width control periods; and
3. Pulse-width modulation in which the error signal is used to control the duty cycle of power continuously applied to the motor.
The present invention generally employs the third-mentioned of these techniques in an improved, cost effective fashion suitable in those environments where the brushless motor is to be driven at a constant speed. U.S. Pat. No. 4,103,216 to Hayes employs both digital time measurement and pulse-width modulation; however, the algorithm used therein necessitates a digital memory, several registers and and arithmetic-logic-unit, which are not needed by the present invention. See also U.S. Pat. No. 4,218,735 to McCutcheon which uses an even more complex microprocessor.